The present invention relates to polycrystalline diamond composite ("PDC") cutters with multiple cutting surfaces used in drag bits for drilling bore holes in earth formations.
PDC cutters have a cemented carbide body and are typically cylindrical in shape. The primary cutting surface of the cutter is formed by sintering a PDC layer to a face of the cutter. Secondary cutting surfaces are formed on the cutter body by packing grooves formed on the cutter surface with diamond and then sintering the diamond to form polycrystalline diamond cutting surfaces.
The cutters are inserted on a drag bit outer body exposing at least a portion of the cutter body and the diamond cutting surface. Typically, the cutter makes contact with a formation at an angle, i.e., the diamond cutting layer is at an angle to the formation surface. As the bit rotates, the PDC cutting layer edge makes contact and "cuts" away at the formation. At the same time portions of the exposed cutter body also make contact with the formation surface. This contact erodes the cutter body surrounding the secondary cutting surfaces, revealing a secondary surface cutting edge or wear surface.
One preferable way to prolong the life of a cutter during drilling, is to increase the hardness of the substrate forming the cutter body. The increase in hardness tends to provide a stiffer or more rigid support for the PDC cutting surface. This will help reduce the magnitude of the tensile stresses in the PDC cutting surface induced by a bending moment during the cutting action, thereby reducing the frequency of cracks in the PDC layer which run perpendicular to the interface. However, a stiffer, harder substrate typically has a lower fracture toughness value and in some cases a lower transverse rupture strength. As a result, once a crack is initiated in the PDC, the substrate is unable to slow the propagation. If a crack is allowed to propagate, it can cause the cutter to fracture and fail catastrophically resulting in the eventual failure of the bit.
Accordingly, there is a need for a cutter having secondary cutting surfaces with an increased resistance to breakage. Moreover, there is a need for a cutter having a stiff, hard substrate supporting the cutter cutting layer for improved cutting but which prevents the propagation of crack growth through the cutter body.